Of the budget 21700 mods I have that are still easy to find
My favourite single 21700 mod is the Dovpo Odin Mini 100, unfortunately even after 2 years from release it tends to be around the £50 price point, can take up to 26mm, but 24 - 25mm still look OK. Decent board..
The Aegis 100 isn't bad, but it's quite heavy and fat as fck so if you don't have Herman Munster hands I'd pass (and the size of the top plate is fcn huge) so smaller tanks look shit, unless fitted with fat boy bubble glass)
The Lost Vape Grus is decent, heavy though but more comfortable and not as fat as the Aegis - again the top plate is massive (30mm I think) so same issue as the Aegis
Aspire Deco.- A bit forgettable. Feels just as heavy as the Aegis and Grus. The silly turn to the side bottom battery cover can tear battery wraps - I open it part way, press the battery in with my thumb, then fully open it. A pain in the arse. Basic board, OK for wattage vaping.
These two I don't own but can be found for around the £30 mark so may be worth a look -
Voopoo Drag X Plus Professional Edition 100W, pretty light for a 21700 mod at 96g and not too porky (don't be confuse it with the Drag X Plus or the Drag X Pro as they have shit boards with no voltage boost)
OBS Engine 100W. Similar style to the Voopoo, 1mm thinner but 10g heavier