I suggest just cross trading. It is extremely easy to get a full load of g5 imperial shielding in about 20 minutes by visiting any medium-high population imperial system and following the steps in this video:
https://youtu.be/lemFjOlEb9c
0. Check nav beacon to find HGE
1. Ensure there will be more than 10 minutes left by time you arrive
2. go there and gobble
3. logout
4. login
5. immediately low wake, but hold zero thruster during countdown (so you dont fly away from HGE)
6. go to nav targets and select the closest nearby unidentified signal source. assuming the countdown didn't expire, it will be the HGE
7. perform 180 pitch turn in supercruise (can be done simultaneously with 6 if you're dextrous)
8. drop out on target (should be in range if you 0% throttled immediately upon SC) and gobble
9. goto 3
You can get most of the other manufactured G5 mats this way, imperial shielding is just easiest to find. I usually fill up on that, Core Dynamics Composites, Military Grade Alloys and one or two other G5s and just cross trade for the rest. See this for the criteria to find; i find it most useful to select all the cells and sort by column B then D:
https://docs.google.com/spreadsheets/d/1Mp7l0bSnMp_G7xWUm75M-XuihDfTdi27rm-vB9K8AX0