First of all, let me explain to you what the Google sandbox is. The sandbox filter is a filter that Google uses on keywords with high search volume and high competition. The whole mindset of the filter is that new website go in some sort of quarantine so they don't rank high in the search engine result pages of Google. This quarantine could take from several months to a year. This was originally introduces to block out spam websites. Before the sandbox filter was born, as a spammer you could make a website, spam Google with it get banned, and instantly make another website and get your original ranking in Google again. But with another domainname. And because Google isn't really fast in giving penalties and bans it takes a couple of months before the new domains get caught. This was easy money making in those days! But not anymore, because now the Google Sandbox filter will solve the problem!
But that doesn't mean we're really stuck to that filter. We are optimizing for search engines, and the sandbox is part of that, so there is a solution.
Only new domains trigger the sandbox filter. So a solution may be to buy an old domain name, or if you already have one, use an old domain name. But this is often really expensive and it doesn't always fit your needs. But there is a solution for that: DeletedDomains.com. On this website are domains that come available the same day. Just search some of the domains for age with the Archive.org wayback machine, and if you find an old domain you can buy it at a hosting company for as little as 15 bucks. You can also take a look in Google to see if the website on the domain is still indexed in Google, that fastens up the process a bit. It doesn't matter what domain you choose, the only thing you have to do is 301 redirect the domain to your main domain. Then you have to hope your old domain get's indexed again so Google will know that the domain moved permanently to the new location. This will give you all the linklove of the old domain and you get the age of that domain. So you can easily build your website and gain high rankings without any sandbox restrictions.