Hire a secure field if you can.
If not, drive to somewhere quiet and not near major roads, don't walk direct from your house as you then leave a trail of sexy bitch scent right to your door which can end up with local dogs escaping (or doing a bunk on walks) to follow it and serenade her from outside.
The middle of the season is the point where shes at her most alluring and receptive so avoid walks then - the week either side of that week/ten days you may get by using anti-mate spray to disguise her scent, but still keep her on lead and avoid places busy with off lead dogs.
I would still spay though, whilst spaying an already 'aggressive' (inverted commas because aggression is simply communication and we all use it...) bitch can make that worse (particularly if timed wrong, it should be 3 months from the start of the last season), thats really only a significant consideration if she lives in a multi-dog household and theres already tensions between the resident dogs.
If she's a solo dog or with another dog and they get on just fine theres no reason to think that won't continue post spay.
The risks of leaving her entire increase season on season and they are significant.
Your vet should be able to give you something like Nutracalm to give before she goes in and then put her first on the list for surgery and send her home ASAP afterwards. I'd also be going for as minimally invasive a spay as possible (ovary only, keyhole) so that recovery time is fast. If your vet is not taking her fear of vets seriously, find another.