How to add a two pole 40A breaker with corner tab(s) to Challenger panel?
Stop faffing with the Challenger breakers, pull out the bottom right 15A breaker, and install a BR115 bottom left and a BR240 bottom right

Buying "new old stock" or newly made Challenger-style (UBI/Connecticut) breakers is an expensive and frustrating exercise, so why are you insisting on trying to go through it? Eaton BR breakers are cross-listed and cross-labeled as type C (or type A for BD/BQ tandem/quadplex breakers) and can be safely and legally used in Challenger panels, as per this letter from UL to C-H (and see my answer here for more details); never mind that a BR240 is $10 and available anywhere in the USA, while the 40A UBI Challenger-style breaker is $50, harder to find, and probably not as good as the Eaton breaker (UBI seems to have...issues with breaker QC and QA).

If you really insist that the bottom left slot stay free, you can get a single, quadplex, BQ2402115 breaker and use it to replace the 15A breaker bottom-right as well as provide the new 40A circuit and a 15A spare.  This is costlier than using the two full-size breakers, but still more reliable than the UBI breaker, and no costlier than the UBI part for that matter.