So you have a 1250w/120v heater strip.
The formula for amperage is watts divided by volts... 1250w/120v=10.42amps, which your 25A SSR theoretically should handle, but the people at Auber Instruments advised me to go with the 40A SSR to avoid any problems. When you buy the next one I would recommend using the 'special' glue that comes with it, they told me why but it was over my head, what I remember is they said to use it liberally.
As for venting why not drill a few holes and cover them with some stainless window screen, you think you have problems with spiders, I have to deal with geckos and they can squeeze into the smallest places.
This is from the Aubers site for their 25A heat sink:
This heat sink is for 25A SSR. It allows the SSR to handle up to 15 Amp without forced air cooling (in an open air environment).
You have 10.4 amps running through the SSR without any ventilation.
When I touched the area where my heat sink is attached it is warm, not so warm to burn, but I can imagine how hot that could become inside of the box if left unvented, which is probably the reason for the SSR failure rather that the amperage.
You probably know this, but when you order the 40A SSR don't forget to order the 40A SSR heat sink, ...the 25A heat sink is too small.
I'm glad I could be of service,