I have two lidless NanoCubes that house shrimp only.
It's also possible that the glass lid for a 30l Dennerle NanoCube would fit (they measure 30x30x35cm, but obviously the height is not relevant here). ProShrimp sells them in the UK and Aquasabi also have them. The provided clips are intended for a tank with curved front corners, but it's easy enough to buy clips for any glass thickness on Amazon.
I’m surprised your guppies are jumping
Are they perhaps being startled?
Given good water conditions (and guppies are quite tolerant - though they do prefer harder water over very soft water) guppies are unlikely jumpers ... though I suppose if tank is filled to the rim, a relatively small movement can lead to “fish over the edge”
I had placed 5 male guppies in an Aspire One 50L tank with a lid, and 3 jumped out over night! There is only a small feeding hole at the front of the lid and a 2 cm gap at the back where the filter pipes go in and that gap is somewhat enclosed with the roots of a Devil's Ivy plant which is going out of the back. I love the guppies as they added colour and life to the tank but never imagined they would jump as they did.