If you are rolling your own it probably doesn't need to be as clever as Anubis is trying to be. Simple something relatively CPU heavy in JS will do the trick, bonus points if you can make it memory-heavy too. Most bots will give up running something CPU heavy currently (though they will run a lot of JS stuff as much of the web is inaccessible without) and they aren't going to optimise your challenge in WASM or similar because it isn't worth it for the small number of sites it protects (Anubis' challenges on the other hand may well be worth optimising for that way because of their wider use, hence that project is stuck in the cleverness war of attrition and definitely needs to move toward memory-hard problems and something more efficient than JS sooner rather than later).

My main reason for planning to implement something like that myself instead of something off the shelf is two-fold:

1. The points mentioned above

2. I can be a dick and make some subsets do harder work (longer challenges, adding time delays that wouldn't phase a bot) - “hi there North American IP, I know how much you like extra hassle hence the love of effectively paying for tarriffs, here, have a little extra work on me!”