Does anyone know the best practices to import older versions of solidity? For example, I want to use a Uniswap V3 interface (0.7.6) with my project that is 0.8.15. What is the best way to do this with foundry?

That should happen easily without workarounds, what's the issue you are facing?

If your question is about the framework, then in Hardhat config there's an option for specifying multiple compilers.

Found a solution on how to integrate uniswap V3 with your solidity 0.8.x projects. Uniswap has a 0.8 branch on the v3-core and v3-periphery repositories. Make a fork of the repositories and make the 0.8 branch the main branch. Works in foundry and on remix with an anvil fork of mainnet.

I usually just make a file containing the relevant parts of the interface

It's easy to upgrade the 2 function signatures you're actually doing to use to a newer solidity version

I just learned that V3's master branch is called 'main'. Would already stop using it just bc of that bullshit

If they do, consider the possibility that your hash might mismatch, e.g. bc metamask only signs hashes of prefixed messages (metamask performs the prefixing and the hashing; the caller has no control over this)

If it's only 3 wallets no need to even code something ad hoc for it

Or you could always make a script where you do that with web3/ethers

But if it's 3 wallets also doing that in the naive way through metamask takes at most 1 minute

English here