Initially, I would base it on an existing rating list, and use the rule of thumb of 70 Elo per factor 2. So to dumb down a 2700 engine to 2000, you would need a time-odds factor of 1024. At long TC that might still be feasible. (40+5 is about 2700 sec, so the stronger engine would get 2.7 sec.) If the time gets shorter than that, it becomes a concern that the alotted time might get lost for a large part on communication delays.
If the engine is too strong or too weak after handicapping, because it lost less / more than 70 Elo per factor 2, the situation will correct itself after a few seasons, as an engine that is too strong will collect too many points, promote to a higher class, and increase its handicap, until it no longer scores enough above average to promote.
The current Knightmate tournament I am running on my
live viewing port has only 4 participants (as I am not aware of any other engines that can play this variant), and I put them in 3 classes, differing by a 12-fold time handicap. Joker KM, in the 1st class, thus gets a 144-fold time handicap when it plays Dabbaba from the 3rd class (12 min against 5 sec).