To stay within your restrictions, here's what I'd recommended.
Pick up the GI sling sold at the Appleseed store.
It's infinitely adjustable to fit " You" and is cheap. Take a black sharpie with you so once you've found your perfect adjustments for sitting and prone you can mark it and then quickly re-adjust it between stages.
You will not have lot of time to mess with them on Sunday once you get roiling on the AQT's. (I use two of them already pre-set).
The sling set up is critcal.
Bring two mags and color one of them on the lip with a red shapie to let you know it's the one with 2 rounds in it.
I have a third spare mag in case one of them malfunctions. That way you can just dump it an continue on.
I lay them out in order on my mat. You will not have to glue them together.
Get an extended mag release. The rapid fire stages will make or break your attempt for a rifleman score. You'll need to dump the empty mag quickly.
Either purchase (cheap) or modify your bolt realese to allow you to just pull back on the bolt to chamber the round.
I have a great soft mat, but if you don't, then get some soft elbow pads. Raw elbows will kill your weekend.
If you want to splurge, get some Tech Sights. Better sights can easily make or break your weekend.
Practice getting into each of your rapid fire positions with your rifle and sling.
Set a timer and practice dropping and loading your mag and getting on target.
The two rapid fire stages are where the wheels fall off for 90% of the shooters.
The marksmanship is not that tough. It's all about your preparation and your proficiency with the two rapid fire stages.