2 Questions: Hatch and Wheels

Hello,

Through a work contact with a local adventure store, I got a tag still on it brand new Cobra Navigator (bare bones) for $350 OTD. Since then, I've modified it quite a bit for spearfishing (I don't hook and line all that often). I put in 2 flush mount holders behind the seats, a fully adjustable Cannon holder at my left foot side rail, an anchor system, dive flag mount, bucket/cooler, etc. However, unlike the one seen in the photo, mine doesn't have the center round hatch.

QUESTIONS:
1) I'd like to install and use the center hatch for storage (mainly safety gear like VHF/GPS, life vest, etc), but the section of my yak (section in photo that has the round hatch) is not flat - it's got a slight concave to it (enough that there is no way a round hatch would sit flat on all sections. Is this normal? Should I avoid installing the hatch?
2) Where I launch from has some shallow stairs. Would a Wheeleez handle stairs better than something else with the larger thinner wheels? I'm debating on what kind of cart I should buy/build. (Note: I don't do a lot of sandy beach launches)

Thoughts/opinions on both would be greatly appreciated.

Could always plastic weld , or use some plastic type epoxy and build up surface area till you get a flush fit... for the hatch as for wheeleze I dunno. They work great in sand.

Have you tried contacting Cobra about the hatch? On the website you show, under contact us is the usa contact number, that would be my first stop,