I have a Domane with down-tube (DT) storage. So, I am familiar with what you're asking. Though I don't know of any alternated hatches, though I've also never looked for any.

Obviously a different door won't make the opening larger. I know you're just asking if there are any 3rd party alternatives, but I'm wondering what you think could exist that would be helpful.

Just as the OEM hatch has a spot for a specific Bontrager multi-tool, it would be possible to have a hatch with an option for a different accessory. But I can't see how that would be helpful for dealing with something that you can't fit into the opening.

The hatch is also where a bottle cage attaches, so any accessories that attach to bottle cage mounts could be used. But that of course is not related to the internal DT storage.

I'm going to guess that fitting in a pump is your problem, since there's nothing else I can think of that is as long as a pump. I get quite a lot into my DT storage including a pump. I use a Lezyne Pocket Drive. I fit it down in the storage area towards the BB. I made a little padded sleeve for it out of neoprene to keep it from rattling and to give me something to reach in and pull it out with. It slips in beside my Di2 battery.

I use the burrito wrap the bike came with as intended with a couple of CO2s and valve, tire levers and a TPU tube. I think I have a tire boot in there as well. In the area just under the hatch I fit a small first aide kit.
I've never needed to get anything out of it on the road. Since I run tubeless, I carry my Dynaplug Race plug tool in my ride wallet in my jersey. At this point, I feel like the things I carry are a talisman. I know as soon as I don't take it, I'll need it.