There is nothing that convincingly replicates spring reverb in a ready to build small package. There is the belton brick stuff which doesn’t sound like a surfy spring reverb. There’s the surfy bear which is fairly large to house the reverb tank. Then there is the amp and guitar of course
Maybe look for a cheap chinese reverb pedal that specifically emulates spring reverb?
The closest thing to the oceans 11 in the DIY realm would probably be the FV-1 dev board from pedalpcb
I dunno if I’m being too picky but to me surf is specifically spring reverb namely the Fender 6G15 reverb.
The vast majority of DIY reverb projects are based on the Belton brick which IMO sounds nothing like a traditional surfy spring reverb. It is more of a 80s digital sound which is cool but not surfy.
The surfy bear is the best and really the only DIY option I know of besides building a straight up 6G15 clone.
There are a few DIY reverb tank driver circuits around but none of them are really known to sound great as far as I know. For surf you really want to smack the reverb tank hard. Most reverb drivers don’t hit as hard as the 6G15