The Pics are taking on Barwell in the third qualifying round tomorrow after beating the Leicestershire side 3-1 in the Southern Premier Central on Tuesday night – and they are hoping for another bumper crowd at Dales Lane.
Rushall have not traditionally been among the highest-supported clubs at step three, but 449 watched their 1-1 draw with Kings Langley last weekend, and McDonald insists they are making a massive difference.
"The supporters have been brilliant. It's not something we've always had, but since the start of the season, it's been phenomenal," he said.
"We haven't had that 12th man in the past, but they've got us points this season by supporting the lads and being in good voice.
"They've been fantastic and probably got us five points in the league, so we'll need it again in the Trophy.
"When they're supporting us like they have done, it gives the lads a big lift.
"They're all talking about it in the changing room, so we need to keep that going and keep everybody entertained, putting in good performances.
"They're helping us massively and there's a real feel-good factor around the club at the minute. It's just a good feeling, and a big part of that is the supporters."
The Pics were backed by 60 travelling fans at Barwell on Tuesday, with Orrin Pendley, Tom Hewlett and Ben Usher-Shipway all scoring in the first half.
"Them having a free weekend last weekend, we always knew they would be fresher than us," said McDonald.
"So, it was important for us to stay in the game, defend properly and maintain our concentration levels.
"It was a good night's work and we blew them away in the first half.
"From the first corner, we scored, and a lot of the balls into the box were dangerous. We played really well, and then in the second half, the team spirit came to the fore.
"We had to defend for a good 20-minute spell and defended well. We knew we had to get three points, and the lads went out and did really well."
McDonald, though, has urged his lads not to get carried away by that result as they look to win and seal £3,750 of Trophy prize money.
"Two games are never the same. We need to approach it right," he added.
"The good things we did on Tuesday we have to do more of.
"It'll be better for us being at home, suit us more in terms of getting it down and playing.
"We just need to make sure we do our jobs, though.
"We stopped their threat a lot on Tuesday, so we will have to do that again."
Also in the Trophy, Stourbridge are hosting Stamford. Hednesford Town are travelling to Corby, and Stafford Rangers are heading to St Ives in Cambridgeshire.
In league action, in the Southern One Central, Halesowen Town are going to Dunstable. Chasetown, in the Northern Premier Division One South East, are taking on Stocksbridge Park Steels at the Scholars Ground.
In National League North, Kidderminster Harriers are looking for their third win on the spin as they go up against Blyth Spartans.