That's a tall order. There's quite a few PL10 GL builds on the boards, but 8 is getting tight. Are you also using the standard 15p/Pl for 120p to start?
As a trainee, you may way to figure out how much less the he can do with the Ring than full-fledged GLs. For instance, Flight is not actually limited by PL (although high-speed Slam Attacks may be a consideration), but there may be some arbitrary limit placed on him for the sake of your game setting. Maybe he's only permitted to use subsonic speed (about Flight 8 ) while based on Earth (which is where I assume you're based), or maybe his Space Travel is limited to only between Earth and Oa. There may be similar limits on the Scanning Beam, Universal Translator, and possibly even the Database until he's proven himself.
The Force Bubble may be an issue. I usually use that as a base power for the array when lowering PL since it's the hardest to reduce, but you may want to simply not have it available for trainees. If you must, though, one way to make a really cheap Bubble is Burst-Affect Others on 9p of Life Support leaving out Starvation & Thirst (the Bubble doesn't serve meals) for 18p and and adding ranks of Reach until it's far enough out for your liking or big enough for your array.
For the rest of it, you'll need watch PL limits. The Force Field will need to drop unless his Dodge/Parry are very low (4 or less). The Force Manipulation powers are tied to Will, so if he has 8 Will, then 8 ranks of Force Blast, Force Constructs and Lifting would work fit comfortably in a 16p Array pool, plus points for each array slot. (Decide whether you want Dynamic as a trainee.) The Mobile Constructs effect, if you choose to have it, would need to be rank 5 to fit at that size. Give him a +8 total attack bonus on his Ring powers and he's good to go.
Of course, Will doesn't have to be at PL; all the established GLs have Will higher than their PL, balanced with a lower Fortitude, so it would be completely reasonable to give your GL trainee a Will of 9 or 10 (no doubt showing serious promise as a natural talent) and raising his Force Manipulation effects. For Will 10, you could have the 3 main slot at rank 10 (blasts as strong as a tank and constructs stronger than steel), Mobile at 6 ranks and even have room for the Bubble with a couple ranks of Reach. His accuracy would have to drop down to +6 overall to balance PL, but the array with all 5 slots Dynamic would only cost 29p total, and that's before the Removable discount.
Think on what you want and I might whip up a full PL8 ring later tonight.
EDIT - actually Saltcrow's build is really good, and 50p is a good target for the cost of the ring.