I have an independent bitvm sidechain model that works without a federation. Instead, there is a "bridge operator" who assists with depositing money to the sidechain as well as with the "happy path" of withdrawing from the sidechain. In my version, you can withdraw even if the bridge operator ceases operations, because there is also a "sad path" that does not require his ongoing cooperation
the main idea is that when you want to deposit money onto the sidechain you should get a "withdrawal contract" from the bridge operator
the withdrawal contract is done in bitvm and it basically says, if you (the withdrawer) can provide a proof of a valid withdrawal request from the sidechain, the prover has up to X blocks to supply proof that he sent you your money on bitcoin
if he does not supply that proof, you may slash him and take the funds that way