nginx config change to send specific file when 404?

Question

i have a pre made nginx config here, which is used to serve files from /usr/share/nginx/html/tiles. so for example i can download url.com/tiles/10/20/30.png when there is a folder and file structure like this in the tiles folder.

if there is no file or folder like this present, it will return a 404 not found.

is there any way to make it possible to send a specific file (tile/fallback.png) if the requested file or folder is not available?

since i really have no idea about nginx configurations, i am struggeling to find the entry point where i would set something like this up.

this is the current config file:

<%
if (s.forceSsl) {
%>
    server {

        listen       80;

        server_name  <%-s.publicDomain%>;

        # Used by Lets Encrypt
        location /.well-known/acme-challenge/ {
            root <%-s.staticWebRoot%>;
        }

        # Used by CapRover for health check
        location /.well-known/captain-identifier {
            root <%-s.staticWebRoot%>;
        }

        location / {
            return 302 https://$http_host$request_uri;
        }
    }
<%
}
%>


server {

    <%
    if (!s.forceSsl) {
    %>
        listen       80;
    <%
    }
    if (s.hasSsl) {
    %>
        listen              443 ssl http2;
        ssl_certificate     <%-s.crtPath%>;
        ssl_certificate_key <%-s.keyPath%>;
    <%
    }
    %>

        client_max_body_size 500m;

        server_name  <%-s.publicDomain%>;

        # 127.0.0.11 is DNS set up by Docker, see:
        # https://docs.docker.com/engine/userguide/networking/configure-dns/
        # https://github.com/moby/moby/issues/20026
        resolver 127.0.0.11 valid=10s;
        # IMPORTANT!! If you are here from an old thread to set a custom port, you do not need to modify this port manually here!!
        # Simply change the Container HTTP Port from the dashboard HTTP panel
        set $upstream http://<%-s.localDomain%>:<%-s.containerHttpPort%>;




location / {
    <%
    if (s.redirectToPath) {
    %>
        return 302 <%-s.redirectToPath%>;
    <%
    } else {
    %>
        <%
        if (s.httpBasicAuthPath) {
        %>
            auth_basic           "Restricted Access";
            auth_basic_user_file <%-s.httpBasicAuthPath%>; 
        <%
        }
        %>

        proxy_pass $upstream;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;

        # CORS Headers
        add_header 'Access-Control-Allow-Origin' '*';
        add_header 'Access-Control-Allow-Credentials' 'true';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';

        <%
        if (s.websocketSupport) {
        %>
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
            proxy_http_version 1.1;
        <%
        }
        %>
    <%
    }
    %>
}

        # Used by Lets Encrypt
        location /.well-known/acme-challenge/ {
            root <%-s.staticWebRoot%>;
        }
        
        # Used by CapRover for health check
        location /.well-known/captain-identifier {
            root <%-s.staticWebRoot%>;
        }

        error_page 502 /captain_502_custom_error_page.html;
        location = /captain_502_custom_error_page.html {
                root <%-s.customErrorPagesDirectory%>;
                internal;
        }
}

I have tried to add a

try_files $uri /tiles/fallback.png;

below the cors lines, but i then only get 500 Internal Server Error nginx. so that might be not the right place or way to do so :/