Glen Boss will perhaps have his best chance in some years to join an elite group with his booking for second favourite Southern Speed in the Caulfield Cup.

The effervescent jockey already has trophies for the Golden Slipper, Cox Plate and Melbourne Cup and will become just the eighth person to complete the grand slam if he can win the Caulfield Cup.

Trainer Leon Macdonald was forced to look for another rider for the defending Cup winner after Craig Williams confirmed his commitment to French horse Dunaden and Boss was first in line.

After missing the spring carnival through injury last year, Boss has announced his return in no uncertain terms with the winners flowing including trebles at Melbourne's past two Saturday meetings.

"Glen ticks every box," Macdonald said.

"He is a big race rider and he's in great form."

Southern Speed finished fourth in Saturday's Group One Underwood Stakes won by New Zealander Ocean Park, who was ridden by Boss and is his mount for the Cox Plate.

Macdonald paid up on Tuesday to keep her in the Cox Plate but said the Caulfield Cup was the preferred option with her lead-up run the Turnbull Stakes (2000m) in which she was fourth last year.

"I paid up for the Cox Plate as a precaution against drawing barrier 20 in the Caulfield Cup," Macdonald said.

"But overall the Caulfield Cup is a better race for her.

"We know she does run the mile and a half (2400m). There's also the colt (Pierro) in the Cox Plate."

The Gai Waterhouse-trained Pierro is the reigning Cox Plate favourite ahead of Ocean Park and More Joyous.

Pierro runs at Moonee Valley on Friday night in the Bill Stutt Stakes ahead of the Caulfield Guineas after which a decision will be made on the Cox Plate.

Boss came within a nose of winning the Caulfield Cup in 2004 when Makybe Diva was second to Elvstroem.

He won three Melbourne Cups on the mare and has two Cox Plates to his credit aboard Makybe Diva (2005) and So You Think in 2009. He won his Golden Slipper in 1995 aboard Flying Spur.