Still the stamped metal covers?
My Dad is running the later cast aluminum ones which SHOULD provide a better clamping surface. Incredibly cheap at a swap meet for an unused pair, like $20. I see them all the time at swapmeets either new or used, for pretty reasonably prices, my guess is maybe they aren't tall enough to clear some of the valvetrain setups people put on crate motors? Can't really explain why people would be getting rid of them otherwise.
And not sure if they sell them for the BBC, but for the centerbolt small block valve covers the Fel-pro neoprene gaskets so far haven't leaked a drop in 10 years. The centerbolts probably seal better than perimeter anyway, but still, I've popped the valve covers off numerous times and never had an issue.