Speaker boxes are best made from 3/4" plywood.
3/4" plywood boxes might not fit there.
Here is the Theile Small Speaker Box engineering Calculator (You have to get the numbers for the calculator data from your speaker manufacturer).:
https://www.micka.de/en/
Here's a thread with the details on who you can call and ask for the recommended box dimensions for the 6x9's you select:
https://techtalk.parts-express.com/forum/tech-talk-forum/54243-6x9-car-speakers-enclosure-volumes
Here's Altec Lansing's database:
Building the best sounding box from wood, is primarily to your advantage.
JBL makes cabinets from special plywood, that equals 3/4" and is lighter and 1/3 the thickness.
You could make a metal box over 1/2" plywood, glued to the boards, which might allow for a thinner box.
You really want the wood in there and possibly some glass to damper the sound.
The issue with 6x9's, they are really designed to be in the back trunk facing the rear window in your car, by design.
They cover this on this link.
https://www.avsforum.com/threads/diy-speaker-enclosure-4-x-6x9s.825165/
It's a good thing you don't want to spend $200 for those metal boxes.
They might not be worth it. Just because they look cool does not mean they sound good.
If you can install Tri-amp or Bi-amp 6" door woofers (which are made for your door volume as a box enclosure, add 1"-2" tweeters pointed towards your front windshield, from the a-pillars, with a power amp and a long box behind the seat sub-woofer, you'll be very pleased with your sound.
You can aim the passenger's side tweeter so it reflects directly at the driver's right ear and aim the left tweeter so it fires off the glass towards your left ear.
The passenger gets whatever she gets... lol.
I would research plastic water proof housings for the door speakers, which keep rain out from the back of the speakers and allow air to breath for mid range and bass response. Some have a flap, which closes and opens to allow the speaker to vent. Some use a sealed back protection cover system with no vent at all.
They are not interchangeable. The speaker is either designed for venting or not.
If you don't care and are using the 6x9's you have, no matter what, then you can try to build a vented box into the same corners of your truck as the steel boxes and use a 1" to 1-1/2" PVC tube vertically up the back corners and cut it down until it sounds good. Your bass will be ported up. Lightly stuff it with glass and push the pvc to within 2" from the bottom of the box. Your mid's will be pointed at your seat cushion. Muffled.
If you run the pvc ports behind the seat horizontally, your bass will come from under the seat, which might be better, so long as the PVC doesn't get broken from tools and stuff back there. 3-way 6x9's are really not the best choice for a 2-door truck cab. You'll over drive the tweeter's to get any mid's/high's, with the speakers pointed behind the seats and burn the tweeter's up in no time.
If you bi-amp and mount tweeters towards the windshield, you'll use 1/10th of the power for highs and use 6x9 mid/base behind the seats?
That might work. You'll still want a sub. 8" 200w min.
Customizing the door panels for audio freaks everyone out. Wiring it correctly is not easy.
Unless you design head rest audio speaker's for your custom bucket seats, you'll have to compromise somewhere.
