Probability of crate types is not yet in the weapon scheme...

Not sure how this counts as "proper" though

The best you can do is make a weapon scheme with all utilities and weapon probabilities disabled.

This will cause only health crates to drop, although it won't be every round, even though you set it to "every round" since what that actually does is roll a health/utility/weapon every round, then pick one of the utilities/weapons according to the probability.

It will in stead spawn a health crate 35% of the time.